On account of the increasing number and size of wind farms, when wind farms are operated on the electrical transmission network it is of increasing importance that they act on the network in a stabilizing manner. By this means, wind farms make their contribution to safeguarding the stability of the electrical network. An important parameter of network quality is the harmonic content. Relevant regulations limit amplitude depending on the order of the harmonics in order to obtain adequate network quality. As a result of the switching processes in the converter, modern converter-controlled wind turbines generate harmonics of different kinds, both those with even and odd order and those with a fixed phase relationship to the network or those with a variable phase relationship to the network. Wind turbines are measured as part of a type test which is to be carried out for each type of wind turbine, and characteristic values are determined therefrom. If a plurality of wind turbines is to be connected to the network, as in the case of a wind farm, as a rule the harmonics are determined from the values for the individual wind turbines with the help of a calculation method. The harmonics emitted by the wind farm as a whole must remain below certain limits. In practice, this often imposes a limit on the wind farm.
When calculating the harmonics for wind farms based on the certified measurements for the individual wind turbine, different procedures are used depending on the type of harmonic: harmonics with low odd order numbers are viewed as typical for machine or mains-commutated converters using thyristor technology. The harmonics are fixed in phase relative to the fundamental, and the harmonics of the individual wind turbines add in a scalar manner. For a farm with N wind turbines, this results in N times the value of a single wind turbine. Higher-order harmonics, in particular in the region of the converter switching frequency, are viewed as typical for freewheeling pulse inverters. The phase relationship of wind turbines equipped with said inverters is not fixed relative to the fundamental, but can drift relative thereto. In fact, the phase relationship is statistically evenly distributed. The values of the individual wind turbines therefore do not add directly; instead, in accordance with the statistical distribution, a proliferation corresponding to the square root of the wind turbines can be applied for these harmonics (100 wind turbines therefore do not generate 100 times the value of a single wind turbine but only 10 times). As this results in a reduction in relation to the approach with low order numbers with scalar multiplication, this is also referred to as a reduction factor of 1/√(N).
Harmonics with low but even order numbers are only inadequately considered in the regulations. As a rule, they contain a significant fixed-phase portion and are therefore a mixture of fixed-phase and variable-phase. The reduction factor 1/√(N) applicable to variable-phase harmonics cannot therefore be used. Conventionally, therefore, a direct scalar addition is carried out without reduction. This is unsatisfactory, as the limits are often very low, as a result of which the size of the wind farm is significantly restricted with regard to the number of wind turbines.
In addition, the diversity of calculation methods makes the application cumbersome depending on the type of harmonic.